Improve Validation - put timestamps of videos

[tnier01]

If the users visits the homepage https://wiediversistmeingarten.org/view/validation they are able to validate bird sightings from randomly selected gardens.
The typically habit of users how to validate the bird is to stop the video of the sighting at the position when the bird can be seen best.

Therefore it would be useful to add to the validation PUT request the timestamp at which the video was paused. This frame could then be used as the first frame for a possible model training.

Starting point for implementation: https://github.com/CookPete/react-player#instance-methods getCurrentTime()